You will be able to approach this side mission after completing Locate Joker in the Steel Mill. During the exploration of the Steel Mill, inside the Assembly Line to be precise, you need to reach the place where Harley Quinn is held #1. Un-gag and listen to what she has to say. You will be able to resume the mission after reaching and securing the Loading Bay #2.
Head to the door in the south-east part of the Loading Bay main room #1. They're locked, so you need to use the Electrical Charge and open the door afterwards, reaching the Freight Elevator #2.
Prepare the Cryptographic Sequencer and interact with the control panel #1. You have to break the password #2 to gain access to two elevator shafts.
Jump into the right shaft #1 and land on the lower ledge. Turn left, equip the Electrical Charge and aim at the engine on the top of the elevator with thugs #2.
It's very important to press RB, so that the charge sends the elevator up #1. Quickly use the newly created passage and jump to the very bottom of the shaft #2.
Look for a higher ledge onto which you can get with the Grapnel Gun. Crouch and carefully approach the two guards #1, taking them out with a single attack from behind. Afterwards use the nearby door (no need for a charge this time) #2 to reach the Boiler Room.
Carefully approach the middle of the Boiler Room, in which you will come across a very numerous group of thugs. Attacking form above unfortunately isn't possible, though you can prepare for the attack by setting traps with the Explosive Gel inside the previous corridor #1, using the Disruptor or by carefully choosing your first target #2.
Throughout the fight itself I'd suggest eliminating the ones equipped with firearms in the first place #1, even though their bullets should hit other enemies more than Batman, considering the size of the room. Look out for the ones throwing fire extinguishers and other large objects #2, evade often.
After the fight, head to the northern part of the Boiler Room and look for a small crate #1. Take the highly useful Freeze Cluster #2, therefore ending the mission.
Even though the mission has ended, it would be good to mention how you can return to the Loading Bay. Get out of the Boiler Room, reaching the Freight Elevator. Place yourself below the elevator with enemies, prepare the Electrical Charge and aim at the engine below the elevator #1. Press RB to make the lift go up and be prepared to use the Grapnel Gun on the previously used upper ledge #2. From there you can easily return to the main part of the Steel Mill.